THERMODYNAMICS OF FLUID-DRIVEN OXIDATION OF URANINITE, PRECIPITATION OF URANOPHANE, AND COPRECIPITATION OF NEPTUNIUM

Recently reported experimental data on the reversed solubility of uranophane and on coprecipitation of neptunium and other estimated thermodynamic properties for uranophane have been interpreted to determine equilibrium constants for the uranyl-neptunyl exchange reaction between uranophane and aqueous solutions. Aqueous speciation calculations for the geochemical environment at the proposed geologic repository for high-level nuclear waste at Yucca Mountain, Nevada, and partial equilibrium reaction path models show that fluid-driven, oxidative alteration of spent nuclear fuel and secondary precipitation of uranophane containing trace neptunium can theoretically limit aqueous neptunium concentrations at nanomolar levels. These results are consistent with neptunium concentrations measured in independent spent nuclear fuel dissolution experiments. Uraninite, a mineralogical analog of spent nuclear fuel, occurs in small, isolated quantities in the uranium deposit at Nopal I, Chihuahua, Mexico. This site bears an uncanny geochemical resemblance to the proposed repository at Yucca Mountain. Reported dating of uraninite at Nopal I provides ages from less than one million years to over thirty million years. However, the vast bulk of primary uraninite in the deposit has been oxidized, mainly to uranophane. Residual uraninite at Nopal I is encased mainly in tough silica cement that effectively isolated it from natural fluid-driven geochemical processes. On artificial exposure to atmospheric conditions, uraninite from Nopal I has been observed to oxidize immediately with transient formation of the mixed valence uranium phase ianthinite. Occurrence of uraninite at Nopal I provides analog evidence for the geologic stability of spent nuclear fuel under conditions that are physically isolated from oxidizing fluids. Alteration of uraninite at Nopal I and formation of secondary uranium minerals are relatively rapid processes driven by oxidizing fluids. Analogous effects can be expected for potential alteration of spent fuel at Yucca Mountain. Thermodynamic, experimental, and natural system studies indicate that interactions between secondary uranophane and groundwater could control the source term for releases of neptunium from the proposed repository at Yucca Mountain.